Drum screen for fertilizer



Oct. 6, 1970 D. A. DUNN 3,532,276

DRUM SCREEN FOR FERTILIZER DONALD A DUNN TYS.

Oct. 6, 1970 D. A. DUNN DRUM SCREEN FOR FERTILIZER 2 Sheets-Sheet 2 Filed April 23, 1968 au A lNvENTDR DDNALD A DUNN f v/Mm M! ATTYs.

United States Patent O 3,532,276 DRUM SCREEN FOR FERTILIZER Donald A. Dunn, Burnsville, Minn., assignor to Cargill, Incorporated, a corporation of Delaware Filed Apr. 23, 1968, Ser. No. 723,533 Int. Cl. B02b 1/02; B07b 9/00 U.S. Cl. 241--77 6 Claims ABSTRACT F THE DISCLOSURE Granular or particulate materials are fed into an upper end of a rotating drum mounted at an inclination to the horizontal and on this upper end of the drum is a screen for passing small particles, fines. Coarser particles of less than a predetermined size flow down the fines screen to and through a coarser screen which has its lower edge at a lesser pitch to the horizontal than does the fines screen. The fines may be drawn through the fines screen by an air stream. Particles larger than the predetermined size flow from the coarse screen to means which comminute the large particles and feed the cornminuted material to another Screener for removing any remaining oversized particles.

This invention relates to screening materials to remove oversized particles therefrom and also to comminuting some of these oversized particles so that they will pass through a separating screen of a given size.

The apparatus is described in connection with the screening of granular fertilizer, but is not limited to this particular material, as the apparatus may be useful for screening various other materials. The screening and comminutng apparatus is particularly useful for granular fertilizers which have been stored or shipped in large quantities; and as a result of such shipping or storing have compacted into large and relatively hard, rock-like masses. Such granular fertilizers are often shipped to a central location and stored in piles which may contain thousands of tons and have a depth of twenty to fifty feet. Fertilizer stored in this manner compacts and forms relatively large clumps which are unsuitable for use by the consumer unless returned to the granular form present before storing.

When it is desired to return the compacted fertilizer to its original granular form, power shovels and hammer mills are used to remove the fertilizer from the pile and to comminute the fertilizer to return it generally into grandular form. While most of the fertilizer returns to grandular form before screening, it is necessary to screen out those particles which were not reduced to the proper size. Typically, such fertilizers are screened on flat vibratory bed screens.

The present invention is directed to screening such granulated materials at a rate which exceeds the rate at which these materials have been screened by fiat vibratory bed screens of the prior art and also to reducing the percentage of material which had been considered unsuitable for use. Additionally, the present invention may be made at a lower cost than the flat bed vibratory Screener.

Accordingly, an object of the present invention is to provide an improved screening apparatus for granulated materials.

Other objects and advantages of the invention will become apparent from the detailed description taken in connection with the accompanying drawings in which:

FIG. l is a side elevational view of an apparatus embodying the invention;

FIG. 2 is a plan view of the apparatus of FIG. l;

FIG. 3 is a diagrammatic, partially sectioned view showing the path of particles moving through a portion of the apparatus of FIG. 1;

3,532,276 Patented Oct. 6, 1970 FIG. 4 is an enlarged, fragmentary sectional View of a hammer mill, which may be used in the apparatus of FIG. l, for comminuting oversized particles; and

FIG. 5 is a sectional view taken along line 5--5 of FIG. 2.

As shown in the drawings for purposes of illustration, the invention is embodied in an apparatus 11 which, very generally, includes an elongated, rotatable drum 13 mounted at an inclination to the horizontal with a charging port 15 at its upper end to receive the material, fertilizer in this instance, which is to be screened and comminuated to a given particle or granular size.

At the upper end of the drum is a fines section 17 for quickly removing and collecting the very fine particles called fines which are, for the most part, particles of such light density as to be capable of being air-borne by a high velocity stream of air. The fines section 17 includes very fine mesh screens 19 across a portion of Which is established a pressure differential, resulting in a suction force for pulling lines through a ductwork 21 to a cyclone separator 23 at which the particles are collected. Most of the incoming material is too coarse to pass through the rotating fines screens and this material flows down and to the left, in FIGS. l and 3, into a coarse section 25 having coarse screens 27 which pass particles having a size equal to or less than that of the `openings therein. Particles larger than the openings in the coarse screens 27 continue to iiow downwardly to the left end of the apparatus as viewed in FIGS. l and 3, and are discharged into an inlet 29 (FIG. 3) of a comminuting means such as, for example, a small hammer mill 31 which reduces these large particles and discharges this reducedinsize material to a secondary Screener 33 (FIG. 1) which will screen particles of the same general size as screen by the primary coarse screen 27.

From an economic standpoint, it is important that as much as possible of the fertilizer be recovered, and significantly, the apparat-us 11 has been found to be, as contrasted to other known screeners, more efiicient in the amount of material recovered. For example, a typical iiat vibratory bed type of screen has a capacity of about tons per hour and a recovery efficiency of about 97%, recovery efficiency being the percentage by weight of the usable fertilizer collected after screening the weight of fertilizer fed into the apparatus. This is in contrast to a 99.77% recovery efficiency experienced for a similar fertilizer with the illustrated embodiment of the invention. Not only is the recovery efiiciency of the apparatus of the invention greater than the recovery efficiency of the flat bed `vibratory separator, but it processes material at a faster rate than fiat bed vibratory screeners, For example, the apparatus 11, which costs considerably less than a iiat bed vibratory Screener, separated 163 tons per hour as compared to 75 tons per hour under similar types of operating conditions.

Proceeding now `with a detailed description of the individual components of the apparatus, the rotatable drum 13 is preferably mounted for rotation in a frame 35 at an upward angle to the horizontal to facilitate the movement of the material through the respective sections of the drum. More specifically, a mounting shaft 37 extends through the drum 13 and has its opposite ends journaled in bearings mounted in blocks 39 (FIG. l) fixed to horizontally extending cross beams 41 at the ends of the frame. The lower, left end of the shaft 37 is connected to and driven by a gear reducer 43 which has an input shaft carrying a pulley 45. Around the pulley is trained a belt 47 `which extends upwardly and outwardly to a motor 49 supported on a bracket 51 fixed to the lower rear corner of the frame 35. It is preferred that the gear reducer 43 step down the rotational speed of the drum 13 and shaft 37 to about 24 r.p.m. in this example.

While the drum 13 is rotating, material is being Jfed into its charging port 15 through a curved, charging hopper 53 which is xed to the upper cross support 41 of the frame 35. Material coming from a belt conveyor (not shown) slides down the hopper 53 and onto the upper end of the rotatable fines screens 19 and then onto the coarse screens 27. In this instance, the fines section 17 and the coarse section 25 are each formed with a plurality of screens 19 and 27 extending longitudinally between and fixed to axially spaced rings 57. Fastened to the rings 57 are radially directed spokes 59 extending inwardly and attached to the shaft 37. Longitudinally extending truss rods 61 extend the length of the drum 13 and between the rings. The rods 61 are angularly spaced from each other to form a skeletal framework or truss. The fines screens 19 and also the coarse screens 27 are carried on the truss rods 61 and spoke-supported rings S7.

Disposed immediately beneath and about the lower half of the fines section 17 is the inlet mouth of the ductwork 21. which is air tight and which includes an upper duct 63 (FIGS. 1 and 3) fixed at its upper edges to inclined, top side beams 65. The walls of this duct 63 extend close to and about the lower portion of the fines section 17 and also define a rectangular cross-sectioned passageway. The passageway extends downwardly to a fianged connector 67 which is joined to another fiange connector and is in Huid communication with an elbow shaped pipe 69. The outlet of the pipe 69 is fixed to the inlet port of the cyclone separator 23.

The cyclone separator 23 includes an internal fan (not shown) for drawing air through the ductwork 21 from the fines section 17. The fan is fixed to a rotatable shaft 71 carrying on its outer end a pulley 73 driven by a belt 75 and a motor 77. The ymotor 77 is supported on a bracket 79 secured to vertical legs 81 of the frame 35 and serves to drive the fan to pull air through the ductwork 21 at a relatively vhigh velocity and rate of flow such as, for example, 5,600 c.f.m, in the described apparatus. A relatively high fiow and |velocity of air is preferred as it will provide sufficient carrying force to bear and transport not only dust-like particles but also the larger fines which pass through screen 19 during operation. At the separator 23, the fines are mechanically separated from the air, which is then discharged into the atmosphere. At an appropriate time, the fines are collected and removed from the cyclone separator. In the prior art flat Screeners, these air-borne particles were, to a large extent, lost. The capturing of air-borne fines results in a higher recovery efficiency, reduced air pollution and better working conditions in the vicinity of the apparatus 11. Also, fine particle material passing with the granular product through the fiat screws of the prior art led to dusty fertilizer products with objectionable application characteristics when the fertilizer 'was spread by the ultimate user.

To prevent accumulations of fines on the fine screens 17 which would substantially blind and impede air fiow through the screen openings, it is preferred to provide sets of Scrapers 85 (FIG. l) which ride on the outer surface of the fines screens 17 and dislodge dust and particle accumulations from these screens. 'In this instance, each Set of scrapers 85 includes three Scrapers 85 of generally block shaped configuration fixed to arms 87 supported on longitudinally extending shafts 89 mounted in upstanding bracket arms 91 fixed to the top side beams 65. In each scraper set, the outer Scrapers are aligned longitudinally (FIG. l) while an intermediate scraper is offset, i.e., spaced circumferential about the screen from these outer Scrapers.

In the preferred and illustrated embodiment of the invention, the cylindrically shaped fines section 17 is about three feet in diameter and about three feet long; the frustro-conically shaped coarse section is about five feet long and tapers uniformly to a right circular cross section at its discharge end which. iS @QU QU@ fOOt ill diameter. The upper left end of the coarse section 25 is three feet in diameter and defines a right circular cross section. This upper end is fixed to the left end of fines section 17, and this right fmsto-conically shaped section 25 tapers uniformly to a one foot diameter at the discharge opening 83 (FIG. 3). The present invention is not limited to any particular size or ratio of lengths of fine section to coarse section to coarse section as these have ibeen varied and satisfactory results have been obtained.

For the fines section 17, good results have been obtained with a slotted, fine screen sold by W. S. Tyler Company of Cleveland, `Ohio and identified as Ty-Rod 9891. The slots are approximately 0.026 inch wide and about one inch in length and the screen is formed of wire having a 0.041 inch diameter and has openings which pass particles about 0.025 inch in diameter. The coarse screens 27 are conventional woven, mesh wire cloth of 0.027 inch diameter wire having substantially square openings about 0.261 inch on a side with about 61.3% of the cloth being open area. The present invention, however, is not limited to these exemplary screens used in the illustrated embodiment of the invention.

It is preferred that the shaft 37 be substantially coaxial with the longitudinal axes of both the fines and coarse sections 17 and 25 of the drum 13 and that the axes of the cylindrically shaped nes section 17 and the frustoconical coarse screen section 25 be aligned with each other. The inclination of the respective coarse and fines screens to the horizontal and relative to each other has a pronounced effect on the rate at which material flows from the inlet opening 15 for the drum 13 to its outlet opening 83 over the hammer mill 31. With a greater inclination to the horizontal, the material fows faster by gravity, but very large inclinations do not allow sufficient time for screening most of the particles through the screens. A good balance of flow rate and screening has been obtained with an inclination of the support shaft 37 for the described embodiment of the invention at about 12 to 16 to the horizontal with 13 being shown. Satisfactory results have been obtained with angles of inclination for the shaft 37 ranging between 10 to 45.

In the illustrated embodiment of the invention, the bottom edge of the fines Screens 19 is parallel to the shaft 37 and, hence, also disposed at an angle of 13 to the horizontal. In this instance, the bottom edges of the conically shaped coarse screens 27 are at an angle of 3 to the horizontal. Thus, there is about a 10 differential between the slope of the bottom surfaces of the respective screens in the illustrated apparatus.

Alternatively, the Shaft 37 has been inclined at a 10 angle to the horizontal, but in such event the ibottom of the coarse screens 27 are then disposed Substantially horizontally and the rate of material ow in the axial direction through the coarse section 25 is reduced to below that desired. In another arrangement having a twenty foot long drum 13, the shaft 37 has been inclined as much as 45 to the horizontal. However, when the shaft is inclined above about 16, the screening is not as good, as a significant amount of the material does not have Suficient residence time on the screen to align with screen openings and to pass through them before being discharged into the comminuting means 31. This decreased residence time can be partially compensated for, and increased contact `between the material and the screens may be obtained, by rotating the drum 13 at faster speeds. Also, it will be appreciated that the 10 angle between the respective lower surfaces of the screens 17 and 27 may be changed. This angle can vary between 0 and 20.

As the granules fall through the coarse screens 27, the particles pass through a discharge duct 93 which is open at its upper end and is disposed adjacent to and generally coextensive with the coarse screens 27. The discharge duct 93 extends longitudinally from the duct 63 to the. adjacent discharge end of the drum 13 and is formed with downwardly converging sidewalls 94 which lead to a discharge opening 95 (FIG. 1) from which the granular material drops from the apparatus 11 and may, in a suitable manner, be collected and removed. The walls 94 of the duct 93 are attached to and supported by the top edges by a pair of top -braces 97 (FIG. 2) which extend downwardly, converge toward one another and are substantially parallel to opposite sides of the conical coarse section 13.

From the discharge opening 83 at the left end of the drum 13, oversized particles fall into the inlet 29 of the hammer mill 31 and are comminuted by the rotating hammers 101 having a hub 103 fixed to a rotatable shaft 105 (FIGS. 1 and 3) which is driven by a belt 107 extending to a motor 109 supported on a horizontally disposed bracket 111 fixed to the short rear vertical legs 81 of the frame 35. The rotating hammers or hars 101 are fastened to the hub and are rotated to break the material to a predetermined size which is small enough to pass through slots between arcuately arranged breaker bars 113. Passing between these breaker bars, the comminuted material falls into a spout 114 leading a fine screen section 115 of the secondary Screener drum 33. The secondary Screener is much smaller in diameter and length than the primary Screener drum 13. The secondary screener drum 33 may be cylindrical in shape throughout its entire length and, in the illustrated embodiment, is about one foot in diameter.

To rotate this secondary drum 32, a motor 117 (FIG. 1) is supported on a bracket 119 which also supports a rotatable shaft 120 for turning the secondary Screener drum. The lower portion of the secondary Screener drum is formed with a coarse screen of the same kind as previously described for the larger coarse screens 27. Particles falling through the respective fine screens or coarse screens fall through a small discharge chute 121 (FIG. 1) which extends downwardly and toward the center of the apparatus to a position adjacent the discharge opening 95 for the large chute. While the amount of material collected from the secondary drum 33 is relatively small in comparison to the amount of material discharged from the primary drum 13, it is nonetheless commercially significant particularly when considering that the rate of material flow is very large, as for example, 163 tons of fertilizer granules per hour. It is to be understood that fines produced by the hammer mill action will flow through the fines screen portion of the secondary drum and are combined with the discharge coming through from its coarse screens to provide the final mixed product in the discharge chute 121. A few of the particles passing through the hammer mill 31 into the rotating secondary drum 33 may still be oversized and fail to pass through the secondary drum screens. These oversized particles are periodically removed and discarded and are counted as a waste product.

To facilitate an understanding of the invention, a brief description of the operation of the screening and comminuting apparatus 11 will be given. Material such as fertilizer which has been broken up in a coarse hammer mill (not shown) is usually delivered by an endless belt conveyor to the inlet hopper 53 at the upper end of the primary screener drum 13. In the illustrated embodiment, the incoming flow is usually continuous from the conveyor belt and at a rate as high as 150 to 200 tons per hour. The fines are pulled downwardly through the fines screens 19 into the ductwork 21 and are carried into the cyclone separator 23 which centrifugally separates the fines from the stream of air and the air is then exhausted.

The fines section 17 of the primary drum 13 is preferred to be shorter and at a steeper angle than the coarse screening section '2S so that the material entering the drum at the fines section flows relatively quickly in the axial direction of the drum from the fines section and onto the coarse screens 27. The high velocity air Cit stream moving to the cyclone separator will rapidly remove these small particles in a short length while the material is flowing downwardly at a relatively fast rate as compared to the rate of flow in the coarse section.

In the coarse section the material flows suciently slowly that a major portion of the granular material has suicient time and contact with the coarse screens 27 to become aligned with a screen opening and pass through it. The bottom edge of the coarse screen is preferably slightly inclined to the horizontal, for example, an angle of 2-5 (degrees), so that material continues to flow by gravity longitudinally and downwardly to the discharge opening 83 (FIG. 3) at the end of the drum 13.

The period of residence of the material on the coarse screens and the amount of contact between particles with the respective drum screens can be changed significantly by varying the angle of inclination of the drum 13 to the horizontal and by varying the speed of rotation of the drum 13. Flow rates of 163 tons per hour and a good screening efficiency and recovery of 99 plus percent have been obtained with the illustrated unit having the shaft 13 at angle 13 to the horizontal and having the bottom edge of the coarse screen at an inclination of three degrees to the horizontal while the drum rotates at a speed of twenty-four r.p.m.

The combination of a cylindrical fines section 17 and a frusto-conically shaped coarse screening section is preferred over a similar drum which is entirely cylindrical in shape for the reason that with a cylindrical drum there is no differential in slope along its bottom edge to obtain the differences in flow rates obtained with the present invention. The same lack of flow rate differentials would also be true in an entirely conically shaped drum for screening.

The rate of flow is such that the particles which fail to pass through the coarse screens 27 are, for the most part, oversized relative to the openings in the coarse screens. These oversized particles are discharged into the inlet of a hammer mill 31 having rotating hammers 101 to comminute the material falling therein. From the hammer mill the comminuted material passes to a secondary rotatable screening drum 33 which also has a fines screen 11S and a coarse screen 123. Relatively small amounts of fines are collected from the material being discharged from the hammer mill, but most of the material comminuted by the hammer mill will pass through the rotating secondary coarse screen 123 into the chute 121 extending to discharge these particles adjacent the opening for the chute 93. In the secondary Screener, good results have been obtained with both the fines and the coarse screen being cylindrical in shape, disposed at an angle of about 3 to the horizontal, and rotating at a speed of about 24 r.p.m. The material will flow down the fines and coarse screens in the secondary drum at substantially the same rate as they are both at about the same angle to the horizontal. The same rate of flow is used as only a relatively small amount of fines are present and without benefit of a suction force to remove the same.

From the foregoing, it will be 'seen that the present invention is particularly adapted to screen and comminute granular material at relatively high production rates and with a very high percent of recovery after screening. As contrasted with flat bed vibratory screens of the prior art heretofore used for screening fertilizer, the present invention was found to be not only faster and more efficient, but also to be less costly in its initial construction.

While a preferred embodiment has been shown and described, it will be understood that there is no intent to limit the invention by such disclosure but, rather, it is intended to cover all modifications and alternate constructions falling within the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. An apparatus for screening granular fertilizer materials comprising a drum having an inlet at one end for initially receiving a charge material to be screened, means mounting said drum at an angle of inclination to the horizontal and for rotating same, a fines screen defining a portion of the periphery of said drum adjacent said inlet, the lower edge of said fines screen being at a predetermined angle to the horizontal, duct means having an inlet mouth disposed about a portion and extending substantially the longitudinal extent of said fines screen, means for generating an air stream to pull said line fertilizer material through said duct means and said lines screen for transporting said nes to a nes collecting station, said duct means being disposed adjacent the periphery of said rotating fines screen and including a duct for directing said air, a coarse screen defining another por- -tion of the periphery of said drum at a location downstream of said fines screen for passing particles of a predetermined size, the lower edge of said coarse screen being at a lesser angle to the horizontal than said lower edge of said fines screen so that gravity exerts a greater force in the direction of material ow on the material flowing along the fines portion than on the material flowing along the coarse portion, comminuting means disposed beneath said drum means for receiving directly said oversized particles of fertilizer therefrom and for breaking said oversized particles, a secondary screening drum having a coarse and ne screen located in a plane below that of said comminuting means for receiving the discharge from said comminuting means and screening the discharge, and means for separating and collecting said ne fertilizer material from said air stream.

2. The apparatus of claim 1 including scraper means 8 for dislodging dust and particle accumulations on said fines screen.

3. An apparatus in accordance with claim 1 in which said means for separating and collecting said fines includes a cyclone separator.

4. An apparatus in accordance with claim 3 in which 'said comminuting means comprises a hammer mill.

5. The apparatus of claim 4 in which the variation in angle between the lower edge of said fines and said coarse screens is less than 20.

6. The apparatus of claim 1 in which said drum has a longitudinal axis disposed between about 10 to 35 to the horizontal.

References Cited UNITED STATES PATENTS FRANK W. LUTTER, Primary Examiner U.S. Cl. X.R. 209-23, 289, 385

Column l Column Column Column Column SEAL) .Attesa UNITED STATES PATENT GFFICE (IIIH'IIFICATE OF CORRECTION Patent; No. 3 u532, 276

Dated October 6%1970 line line line line line line drum In the references:

EdwanllLFletchmI.

Anestng Officer Inventods) Donald A. Dunn It 1s certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

"granular" is misspelled;

"granular" is misspelled;

l2, "commnuted" is misspelled;

8, "to coarse section" is shown twice;

29, secondary drum "32" should be secondary -33;

Beardsley Patent 2,389,715 shows an issue date of 1935 in the patent. correct year is 1945.

The

SXGNED MWD SEALED MARZ 197| www E. JR. @omissioner or Patents F ORM POI O50 (10-69) 

